1. Wet Bulk Micromachining –
a STIMESI II tutorial
Per Ohlckers, Vestfold University College
Daniel Lapadatu, SensoNor Technologies

2. Outline 1. Background and Motivation 2. The Silicon Crystal
3. Isotropic Wet Etching
4. Anisotropic Wet Etching
5. Selective Etching
6. Convex Corners

3. Outline 1. Background and Motivation 2. The Silicon Crystal
3. Isotropic Wet Etching
4. Anisotropic Wet Etching
5. Selective Etching
6. Convex Corners

4. Manufacturing Processes
Serial (e.g. Focused Ion Milling - FIB) vs. batch (e.g. bulk Si micromachining) vs. continuous (e.g. doctor’s blade).
Additive (e.g. evaporation) or subtractive (e.g. dry etching).
Projection (almost all lithography techniques) vs. truly 3D.
Mold vs. final product.
FIB
There are many different techniques that are being used.
However, in general, batch processing is the most powerful technique and used mostly.

5. Micromachining
Bulk Micromachining is a process that produces structures inside the substrate by selective etching.
Surface Micromachining is a process that creates structures on top of the substrate by film deposition and selective etching.
Surface Micromachining
Bulk Micromachining

6. Classification of Bulk Silicon Etching
Bulk Micromachining is a process that produces structures inside the substrate by selective etching.
Wet Etching
Dry Etching
Crystal orientation
dependent
Process
dependent
Isotropic
Anisotropic
Isotropic
Anisotropic
Acidic Etchants
Alkaline Etchants
BrF3 XeF2
F-based plasmas

7. Etching Features Etch rate: Etch selectivity: Etch geometry:
Rate of removal of material/film.
Varies with concentration, agitation and temperature of etchant, porosity and density of etched film.
Etch selectivity:
Relative etch rate of mask, film and substrate.
Etch geometry:
Etching depth (R);
Mask undercut (U);
Slope of lateral walls (S);
Bow of floor (B);
Anisotropy.

8. Bulk Silicon Etching: Examples
Deep cavity by wet,
anisotropic etching
Release etch
by RIE
Recess etch
by RIE

9. Wet Silicon Etching: Examples
Isotropic etching with HNA
(HF : Nitric Acid : Acetic Acid)
Anisotropic etching with KOH
(110)
(100)

10. Outline 1. Background and Motivation 2. The Silicon Crystal
3. Isotropic Wet Etching
4. Anisotropic Wet Etching
5. Selective Etching
6. Convex Corners

11. Structure of Single Crystal Silicon
Face-centred cubic (fcc) structure (diamond structure) with two atoms associated with each lattice point of the unit cell.
One atom is located in position with xyz coordinates (0, 0, 0), the other in position (a/4, a/4, a/4), a being the basic unit cell length.
Lattice constant a = 5.43 Å.
The arrangement of the silicon atoms in a unit cell, with the numbers indicating the height of the atom above the base of the cube as a fraction of the cell dimension.

12. Miller Indices
Miller indices are a notation system in crystallography for planes and directions in crystal lattices.
A lattice plane is determined by three integers h, k and l, the Miller indices, written (hkl). The indices are reduced to the smallest possible integers with the same ratio.
Determining the Miller indices for planes by using the intercepts with the axes of the basic cell.

13. Determining Miller Indices
Example:
Take the intercepts of the plane along the crystallographic axes, e.g. 2, 1 and 3.
The reciprocal of the three integers are taken: 1/2, 1/1 and 1/3.
Multiply by the smallest common denominator (in this case 6): 3, 6 and 2.
The Miller indices of the plane are: (362).

14. Crystallographic Planes and Directions
(abc) denotes a plane.
{abc} denotes a family of equivalent planes.
[abc] denotes the direction perpendicular on (abc) plane.
<abc> denotes a family of equivalent directions.
{100}, {110} and {111} are the most important families of crystal planes for the silicon crystal.

15. Single Crystal Silicon Wafers
Primary and secondary flats indicate the dopant type and surface orientation.
Wafer diameter in current fab standards: from 100 to 300 mm.
Wafer thickness in current fab standards: from 250 to 600 µm.
Surface orientation:
(100) for MOS and MEMS;
(110) for MEMS;
(111) for bipolar.
Secondary flat
Primary flat

16. Orientation of flats for 100 mm wafers
Standard 100 mm Wafers
The position of the flat(s) indicates the surface orientation and the type of doping.
The primary flat on (100) and (110) wafers is along the [110] direction.
Orientation of flats for 100 mm wafers
p-(111)
n-(111)
n-(100)
p-(100)
Secondary flat
Primary flat

17. Wafers Used in MultiMEMS
P-type, 150 mm Si wafer.
(100) ± 0.5º Surface.
[110] ± 0.5º Primary Flat.
Si Wafer

18. Outline 1. Background and Motivation 2. The Silicon Crystal
3. Isotropic Wet Etching
4. Anisotropic Wet Etching
5. Selective Etching
6. Convex Corners

19. Isotropic Wet Etching of Silicon
All crystallographic directions are etched at the same rate.
Features:
Etchants are usually acids;
Etch temperature: °C;
Reaction is diffusion-limited;
Very high etch rate (e.g. up to 50 µm/min);
Significant mask undercutting.
Masking is very difficult:
Au/Cr or LPCVD Si3N4 is good.
SiO2 may also be used for shallow etching.
with stirring
without stirring Si

20. Isotropic Etching of Silicon: EtchantsHF
HNO3
CH3COOH + H2O
10 ml
30 ml
80 ml
25 ml
50 ml
9 ml
75 ml
Etchant
(Diluent)
Typical
Composition
22 °C
Temp. 40
7.0
Etch Rate [µm/min]
Mechanism
oxide removal:
HNO3 + H2O + HNO2 → 2HNO2 + 2OH– + 2h+
Si4+ + 4OH– → SiO2 + H2
SiO2 + 6HF → H2 SiF6
hole injection:
oxidation:

21. Silicon Etching with HNA
HNA: mixture of
49,23% HF,
69,51% HNO3 and
acetic acid (CH3COOH) or water (H2O) as diluent.
HNO3 oxidizes the silicon, HF removes the oxide:
High HNO3:HF ratio, - Etch limited by oxide removal.
Low HNO3:HF ratio - Etch limited by oxide formation.
Dilute with water or acetic acid:
CH3COOH is preferred because it prevents HNO3 dissociation.
(µm/h)
Iso-Etch Curve (from Robbins et al.)

22. Isotropic Etching of Glass
Single- or double-side etching of glass wafers is achieved either by using HF-water solution or HNA.
Typical etch rate for borosilicate glass in HNA: 1.9 µm/min.
Applications:
Etching cavities and through-holes;
Etching gas/fluid channels.
Undercut
Cavity
Through-hole
Mask

23. Outline 1. Background and Motivation 2. The Silicon Crystal
3. Isotropic Wet Etching
4. Anisotropic Wet Etching
5. Selective Etching
6. Convex Corners

24. Anisotropic Wet Etching of Silicon
Crystallographic directions are etched at different rates.
Features:
Etchants are usually alkaline;
Etch temperature: °C;
Reaction is rate-limited;
Low etch rate (ca. 1 µm/min);
Small mask undercutting.
Masking is very difficult:
LPCVD Si3N4 is good;
SiO2 may also be used with some etchants. Si

25. Etching Setup Laboratory setup for wet chemical etching of silicon.
The principles for industrial manufacturing equipment are the same.

26. Anisotropic Etching of Silicon: Etchants
Etylenediamine
Pyrocathecol
Water
750 ml
120 gr
100 ml
115 °C
0.75
35:1
SiO2
Si3N4
metals
Etchant
(Diluent)
Typical
Composition
Temp.
Etch Rate
[µm/min]
Etch Ratio
(100)/(111)
Masking
Film
240 ml
1.25
KOH
Water + Isopropyl
44 gr
85 °C
1.4
400:1
TMAH
220 gr
780 ml
90 °C
1.0
100:1

27. Chemistry of Anisotropic Etching
Etching phases:
Transport of reactants to the silicon surface;
Surface reaction;
Transport of reaction products away from the surface.
Key etch ingredients:
Oxidisers;
Oxide etchants;
Diluents and transport media.
Mechanism
Si + 2OH– → Si(OH) e–
4H2O + 4e– → 4OH– + 2H2 (gas)
Si(OH) OH– → SiO2(OH)22– + 2H2O
Overall:
Si + 2OH– + 2H2O → → SiO2(OH)22– + 2H2 (gas)

28. Anisotropic Etching of (100)-Si
Cavity defined by:
{111} walls
slow-etching planes;
{100} floor
fast-etching plane.
Final shape of cavity depends on:
Mask geometry;
Etching time.
Shape of cavity:
Truncated pyramid;
V-groove;
Pyramid.

29. Cavity Geometry for (100)-Si
Anisotropically etched cavity in (100) silicon with a square masking film opening oriented parallel to the <110> directions.
Wb = W0 – 2·l·cot(54.7°)

30. Mask Undercutting (a) is a pyramidal pit bounded by the {111} planes.
(b) is a type of pit expected from slow undercutting of convex corners.
(c) is a type of pit expected from fast undercutting of convex corners.
In (d), further etching of (c) produces a cantilever beam suspended over the pit.
(e) illustratates the general rule for undercutting assuming a sufficiently long etching time.

31. Anisotropic Etching of (110)-Si
Cavity defined by:
{111} walls
slow-etching planes;
{110} floor
fastest-etching plane;
{100} bottom side walls
fast-etching planes;
Final shape of cavity depends on:
Mask geometry;
Etching time.
Cavity shape:
Rhombic prisms;
Hexahedric prisms.

32. Cavity Geometry for (110)-Si

33. Outline 1. Background and Motivation 2. The Silicon Crystal
3. Isotropic Wet Etching
4. Anisotropic Wet Etching
5. Selective Etching
6. Convex Corners

34. Methods for Selective Etching
Time etching methods:
Calculate the needed etching time on the basis of the etching rate.
Easy, but inaccurate method, as etching rate varies with the chemical condition of the etchant and geometrical factors limiting the agitation of the etch. Typical accuracy: ± 20 µm.
Inspect the depth of the etched cavity in appropriate time intervals until desired depth is reached.
Time consuming, but improved accuracy. Uneven etching depth from cavity to cavity due to chemical and geometrical factors is still a problem. Typical accuracy: ± 10 µm.
Chemical selective techniques:
The etching stops when a chemically resistive layer is reached.
Typical accuracy: ± 3 µm.
Electrochemical selective techniques:
The etching stops on reverse biased pn junctions.
Typical accuracy: ± 1 µm.

35. Time-Stopped Etching
Example of etching stopped at an arbitrary depth, exhibiting a flat floor.

36. Etching Stopped by {111} Walls
Example of etching stopped by the intersecting {111} walls, exhibiting a pyramidal groove.

37. Boron Etch-Stop Technique
Chemical selective etching:
Etch rate depends on boron concentration.
Etching stops if boron concentration exceeds 5·1018 cm–3.
Boron stop layer is manufactured:
By diffusion deposition, implantation or both;
On the opposite surface of the wafer with respect to the etch cavity.
Boron-dependent etch rate of silicon (from Seidel et al.)

38. Boron Etch-Stop Mechanism
Interstitial bonds require more energy to be broken.
The electrons supplied by the etchant recombine with the holes in the bulk, rather than participating in the chemical reaction.
Silicon atom
Substitutional
boron atom
Interstitial

39. Boron Etch-Stop Shortcomings
Electronics cannot be integrated in the boron stop layer.
Solution: depositing an epitaxial layer atop the stop layer, with appropriate doping as substrate material for integrated devices.
Controlling the autodoping of the epi-layer is challenging.
n-type epitaxial layer
p+ type boron stop layer
n-type substrate

40. Electrochemical Etch-Stop (ECES)
Electrochemical selective etching:
Etch rate depends on the applied potential.
Etching stops if the applied potential exceeds a threshold value, called passivation potential.
Low-doped material, both p- and n-type, can be passivated:
To be used as substrate for integrated components such as piezoresistors.
High accuracy, typically ± 1 µm:
Achieved by using well-controlled implantation and diffusion techniques.
KOH and TMAH can be used:
Both avoid the health dangers of EDP.

41. Wafer Holder for ECES
A practical way to make a wafer holder to be used for electro-chemical selective etching.
The principles for industrial manufacturing equipment are the same.

42. Electrochemical Etch-Stop Mechanism
Etch-stop achieved by reverse biasing the pn junctions.
More in the Bulk Silicon Etching tutorial...

43. ECES for MultiMEMS
Electrochemical etch-stop allows 3 different thicknesses:
Full-wafer thickness (400 µm)
For heavy seismic masses;
Epi-layer thickness (3 µm)
For thin membrane, springs;
N-well thickness (23 µm)
For thick membranes, masses, bosses…

44. Etch-Stop on Multi-Level Junctions
Thin Membrane
Very Thin Membrane
Thick Membrane (Mass/Well)

45. Outline 1. Background and Motivation 2. The Silicon Crystal
3. Isotropic Wet Etching
4. Anisotropic Wet Etching
5. Selective Etching
6. Convex Corners

46. Undercutting of Convex Corners
High etch-rate of high-index planes:
Severe undercutting of convex corners;
Truncated pyramids or V-grooves as final cavities.
Mask will be undercut until {111} planes are exposed.
Mask
{111}

47. Compensation for Convex Corners
Etching without corner compensation structure
Etching with corner compensation structure
Corner Compensation in Silicon (from Gupta et al.)
Corner compensation of mask is difficult to establish as a repeatable process; highly dependant on etching parameters.

48. Corner Compensation Structures
Desired Result
A simple approach to convex corner compensation
(from Wei Fan et al.)

49. Designing with Undercut Corners
Fabrication of MEMS - MEMS Technology Seminar (from Burhanuddin Yeop Majlis)

50. Thank you
for your attention !